1. Field of the Invention
The present invention relates to electronic devices such as light-emitting diodes and field-effect transistors (FETs) including high-electron-mobility transistors (HEMTs). The present invention particularly relates to an electronic device containing group-III element nitride semiconductors.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2002-359255 (hereinafter referred to as Patent Document 1) discloses a technique that a base layer, containing a nitride semiconductor containing aluminum (Al), having a thickness of 0.5 to 100 μm is formed on a conductive silicon carbide (SiC) substrate at a temperature of 1100° C. to 1250° C., an undoped gallium nitride (GaN) layer is formed on the base layer, and a nitride semiconductor layer is formed on the GaN layer. In the technique, the base layer is used to insulate the substrate. The base layer most preferably comprises aluminum nitride (AlN) which has a wide bandgap and which is insulative.
In conventional techniques, base layers and/or GaN layers disposed on the base layers are doped with a transition metal, such as iron (Fe), for forming a deep impurity level, or doped with carbon (C) in some cases such that these layers are rendered insulative.
In the technique disclosed in Patent Document 1, there is a problem in that although the device is insulated from the substrate, when the base layer is made of the nitride semiconductor having Al as an essential element, an electronic device prepared by the technique has low dielectric strength and a leakage current flows between an input and an output terminal when the electronic device is out of conduction or is turned off. In the conventional techniques, there is a problem in that device properties are deteriorated due to factors below.    (1) While crystals in undoped layers are being grown or after the crystals are grown, the dopant (for example, the transition metal) migrates to undoped layers; hence, the crystallinity of the undoped layers and that of semiconductor layers disposed on or above the undoped layers are deteriorated.    (2) If the dopant (for example, the transition metal) is transported to regions by thermal diffusion or the like, located near heterojunction for forming two-dimensional electron gas layers, the resulting dopant causes electron scattering in areas on channels, thereby increasing the on-resistance of electronic devices including the layers containing the dopant.